Cause The OSU Plant Clinic has identified 4 different leaf spotting fungi from past samples of mountain ash including Cercospora, Cladosporium (leaf scorch), Gloeosporium, and Septoria spp. not including rusts. Septoria sitchensis has been reported from all states while Diplocarpon mespili has been reported from WA and ID and Phyllosticta globigera and Venturia inaequalis have been reported from Washington. Most fungal leaf spots are favored by wet weather and/or overcrowding that keeps leaves wet for extended periods of time. Airborne or water-splashed spores infect young leaves during wet weather, often in spring. They generally survive unfavorable conditions in plant debris.
Symptoms Phyllosticta - Round to irregular, brown spots form on leaflets. Tiny, black fruiting structures form within older spots.
Cultural control
- Avoid overhead irrigation that keeps foliage wet for extended periods.
- Rake and destroy fallen leaves. Remove and destroy infected leaves from plants where practical.
- Space plantings and prune to improve air circulation.
Chemical control Begin sprays at budbreak and continue at regular intervals during wet weather.
- Avelyo at 3 to 5 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Badge X2 at 1.5 to 2 lb/A. Group M1 fungicide. 48-hr reentry. O
- CuPRO 5000 at 1.5 to 5 lb/A but only up to 2 lb/A when new growth is present. Group M1 fungicide. 48-hr reentry.
- According to IR-4 data, Heritage is safe on some species of this genus. Use at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker on a few plants first before wide spread use. Group 11 fungicide. 4-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Phyton 27 at 2.5 to 4 oz/10 gal water. Group M1 fungicide. 48-hr reentry.
Reference Farr, D.F., Bills, G.F., Chamuris, G.P., and Rossman, A.Y. 1989. Fungi on plants and plant products in the United States. APS press.